Simian virus 40 small t antigen cooperates with mitogen-activated kinases to stimulate AP-1 activity.

Abstract

The simian virus 40 small tumor antigen (small t) specifically interacts with protein phosphatase type 2A (PP2A) in vivo and alters its catalytic activity in vitro. Among the substrates for PP2A in vitro are the activated forms of MEK and ERK kinases. Dephosphorylation of the activating phosphorylation sites on MEK and ERKs by PP2A in vitro results in a decrease in their respective kinase activities. Recently, it has been shown that overexpression of small t in CV-1 cells results in an inhibition of PP2A activity toward MEK and ERK2 and a constitutive upregulation of MEK and ERK2 activity. Previously, we have observed that overexpression of either ERK1, MEK1, or a constitutively active truncated form of c-Raf-1 (BXB) is insufficient to activate AP-1 in REF52 fibroblasts. We therefore examined whether overexpression of small t either alone or in conjunction with ERK1, MEK1, or BXB could activate AP-1. We found that coexpression of small t and either ERK1, MEKI, or BXB resulted in an increase in AP-1 activity, whereas expression of either small t or any of the kinases alone did not have any effect. Similarly, coexpression of small t and ERK1 activated serum response element-regulated promoters. Coexpression of kinase-deficient mutants of ERK1 and ERK2 inhibited the activation of AP-1 caused by expression of small t and either MEK1 or BXB. Coexpression of an interfering MEK, which inhibited AP-1 activation by small t and BXB, did not inhibit the activation of AP-1 caused by small t and ERK1. In contrast to REF52 cells, we observed that overexpression of either small t or ERK1 alone in CV-1 cells was sufficient to stimulate AP-1 activity and that this stimulation was not enhanced by expression of small t and ERK1 together. These results show that the effects of small t on immediate-early gene expression depend on the cell type examined and suggest that the mitogen-activated protein kinase activation pathway is distinctly regulated in different cell types.The stimulation of quiescent cells with growth factors induces widespread changes in gene expression, including the activation of promoters controlled by the transcription factor AP-1 (30). Following the binding of growth factors to cell surface receptors, a protein kinase cascade is activated, which generally results in the activation of two kinases termed ERK1 and ERK2. ERK1 and ERK2 belong to a family of kinases known as extracellular signal-regulated kinases (ERKs) or mitogen-activated protein (MAP) kinases (12). Both ERKI and ERK2 are activated by phosphorylation on specific threonine and tyrosine residues (44). This phosphorylation is catalyzed by a threonine/tyrosine kinase known as MEK (MAP and ERK kinase) (15,16,53,54,57). MEK itself is activated by phosphorylation on uncharacterized serine and/or threonine residues by both c-Raf-1 or MEK kinase (17,28,31,32,58 ing threonine in ERK1 and ERK2, and the activating serines and threonines in MEK1 (1,4,7,22,23,27,28,31). It is not entirely clear, however, how PP2A impacts this cascade in vivo. Indirect evidence s...

Spatial regulation of Raf kinase signaling by RKTG

“…Various constitutively active components of the Ras/Raf/MEK pathway were used to activate Elk-1 transactivation. They included H-Ras-V12, Raf-1(BXB), which only contains the Raf-1 kinase domain (26), Raf-1(CAAX), which is constitutively targeted to plasma membrane by fusing Raf-1 with the C terminus of H-Ras (27), and MEK1(DD) (28). As shown in Fig.…”

Simian virus 40, poliovaccines and human tumors: a review of recent developments

“…Small t shares 82 aminoacids at its aminoterminus with Tag; the remaining 92 amino acids are unique. Small t enhances the transforming capacity of SV40 by increasing the production of Tag (Bikel et al, 1987); by contributing to the complete inactivation of cellular p53 (Tieman et al, 1995); and by stimulating mitosis in quiescent cells (Cicala et al, 1994), an e ect that may be mediated by the ability of small t to induce AP-1 (Frost et al 1994). In 1979, Lewis and Martin demonstrated that small t mutants injected subcutaneously (sc) into hamsters induced sarcomas with a prolonged latency compared to wild-type (wt) SV40 (Lewis and Martin, 1979).…”

A role for the small GTPase Rac in polyomavirus middle-T antigen-mediated activation of the serum response element and in cell transformation

“…The phosphotyrosines act as docking sites for SH2 and PTB domain-binding cellular proteins like the lipid kinase phosphatidylinositol 3-kinase (PI 3-kinase) (Whitman et al, 1985), the adaptor protein SHC Campbell et al, 1994) and phospholipase C-g1 (PLC-g1) (Su et al, 1995). In addition, the serine/threonine speci®c phosphatase 2A (PP2A) (Walter et al, 1990;Pallas et al, 1990) and a member of the 14-3-3 family of proteins have been found to associate with middle-T. PP2A, a ubiquitously expressed cellular phosphatase, is also known to bind to SV40 or polyoma small-T and has been shown to interfere with downregulation of ERK1 and ERK2 (Sontag et al, 1993;Frost et al, 1994). C-Src as well as 14-3-3 have been implicated in the activation of Raf (Marais et al, 1995;Freed et al, 1994;Irie et al, 1994;Fu et al, 1994;Li et al, 1995).…”